1. Field of the Invention
The present invention is in the field of manufacturing tape-shaped silicon bodies such as those employed in solar cells and relates specifically to an improved method and apparatus for applying controlled amounts of silicon to a reticulate body such as a mesh of a refractory material at improved speeds.
2. Description of the Prior Art
Coatings of carrier bodies containing molten silicon are known, for example, from German OS No. 30 10 557 A1 (U.S. Ser. No. 238,943, now abandoned) or from Abstract No. 327 by C. Belouet from the Electrochemical Meeting at St. Louis, May 11 through 16, 1980. In the first mentioned method, the coating is conducted relative to the drawing rate such that because of the high surface tension of the molten silicon, a thin layer less than 150 .mu.m thick is formed in the meshes of the carrier body which consists of graphite or graphitized silica glass threads. The tape-shaped carrier body is conducted through a gap-shaped opening in a channel extending in the direction toward the melt surface and which is situated in the floor of a vat containing molten silicon. The gap-like opening is matched to the dimensions of the tape-shaped carrier. While it is possible to achieve uniform coating by means of this apparatus, the apparatus is only employable on a laboratory scale. The same is true of the method illustrated in FIG. 4 of the reference wherein the carrier body is moved past an opening disposed in the floor of the melt crucible, being moved past the same perpendicular relative to the opening and at a slight distance therefrom.
The second-named reference (Belouet) discloses a device wherein a carrier body consisting of a graphite ribbon is conducted through an opening situated in the floor of the melt crucible. This device has the danger that molten silicon will escape through the opening in the crucible floor and that the coating will thereby be interrupted.
An apparatus of the type initially discussed wherein a spilling of the molten silicon is suppressed has been proposed in German patent application No. P 32 17 686.4. This device is different from the prior art in that the coating location is isolated from the melt supply, and a higher thermal stability is thereby achieved at the drawing location. The transport of the carrier body occurs from bottom to top through a guide slot.
For a usable coating of a carrier body with silicon, there must not only be a problem-free drawing of the body through the molten silicon but, in addition, the geometry of the silicon meniscus at the exit location of the carrier body from the silicon melt should be controlled. Because the geometry of the meniscus can be varied within certain limits by means of a suitable design of the drawing nozzle, certain fundamental conditions such as the maximum meniscus height when drawing the carrier body from bottom to top can be varied only within very narrow limits. A fundamental limitation therefore exists which frequently can have a disadvantageous effect on the coating behavior.
A method for drawing a polycrystalline silicon layer on a carbonized ceramic substrate is known from German OS No. 30 36 104 A1 (U.S. Pat. Nos. 4,251,570 and 4,252,861) wherein the substrate is extracted from the silicon melt in an angle deviating from the vertical. The heating thereby takes place through the lower side which is still immersed and has not been coated. The growth rate of the silicon layer is increased by means of drawing obliquely up through the melt. An oblique drawing of surface silicon, however, is very involved and presents problems in terms of technology, particularly when the drawing is combined with an after-heating zone which is indispensable for fast drawing and for a high crystal quality of the silicon.